The present invention relates to inkjet printing apparatus, such as printers, copiers, facsimiles and the like, and more particularly it is concerned with a holddown device for the paper or medium being printed in this kind of apparatus.
Inkjet printing apparatus, and inkjet printers in particular, are provided with systems or devices, which will be referred hereinafter as holddown devices, that keep the paper flat while it is being printed by a travelling inkjet printhead.
The design of a holddown device to keep the paper flat at least in the print zone of the apparatus must deal with a number of contrasting issues.
On one hand, for instance, the distance between the printhead and the paper must be as small as possible, for example less than 1.7 mm, in order to obtain an accurate positioning of the ink dots projected from the printhead and to avoid spraying artifacts.
However, due to the water content of the ink, the paper is subject to a phenomenon known as xe2x80x9ccocklexe2x80x9d consisting in the swelling and expansion of the paper during the printing operation, such that the paper forms bubbles and wrinkles and as a result the distance between the paper and the printhead decreases in some areas. Cockle can cause two major drawbacks: first of all, the risk of ink smearing or paper crash because the printhead touches the paper, and further the appearance of visible defects in the printout, known as xe2x80x9cvertical bandingxe2x80x9d, because due to the presence of a bubble the ink dots fall in points offset from their correct position, e.g. all displaced towards the same side, leaving visible marks on the plot in the form of parallel lines.
Some devices known in the art provide a negative air pressure under the medium in order to maintain it flat in the print zone.
One example of such a vacuum holddown device is described in EP-A-0 997 302. This device includes a platen, on which the paper is kept flat, which partially overlaps the paper drive roller. A plurality of grooves, all connected to a vacuum source, are formed in the platen, the aim of said grooves being to extend the vacuum and therefore the holddown action towards the drive roller, in order to allow more accuracy in the printing operation while keeping at the same time the drive roller out of the vacuum system.
In practice, in order to control the cockle effect in high quality printing this vacuum holddown system requires the provision of overdrive wheels or a similar tensioning device in the front part of the platen, i.e. downstream of the printing zone, in order to tension the paper in the feeding direction while it is being printed. This solution makes the holddown system complicated, and its cost is quite high.
Other solutions, such as heaters or fans to dry the media during printing, have high power requirements and safety problems.
Increasing the vacuum to reduce cockle is also not a good solution, because higher vacuum levels increase the cost, bring about noise problems and the risk of creasing the paper, and also hinder the advance of the media being printed.
The present invention seeks to provide an improved inkjet printing apparatus, having a holddown device simpler in construction and lower in cost with respect to the prior art which can successfully neutralise the effects of wet cockle in the print zone.
Accordingly, the inkjet printing apparatus of the present invention is provided with a holddown device for a medium lying on a media supporting platen on which a print zone is defined, and is characterised in that said holddown device comprises first cockle-control means which, at least in a medium output zone arranged downstream of the print zone in the direction of advance of the medium, control an expansion of the medium to be in the form of a wave defined by a plurality of bubbles, said wave being substantially adapted in frequence to a ridged surface of the supporting platen.
By controlling the shape of the deformed medium, the invention successfully achieves a reduction of the height of the wrinkles or bubbles caused by cockle. This effect is achieved by forcing the medium to adopt a wave form that xe2x80x9ccopiesxe2x80x9d the underlying support platen, which has a ridged surface, i.e. a surface having a succession of incuts and projections.
Advantageously, said wave generated in the output zone is induced to reproduce upstream towards the print zone.
Thus, the bubbles are generated outside the print zone and are induced to propagate towards and partly into the print zone; this controlled generation and propagation avoids the negative effects of free expansion of the paper due to cockle in the print zone, and the bubble height is kept small.
With this reduction of height of the bubbles, the risk of contact of the medium with the printheads is thus much lower than if the paper expands in a free shape.
This makes the printer according to the invention particularly suitable for applications in which it is especially important to avoid down times and non-programmed maintenance operations.
The cost of the holddown system is significatively lower than in prior art solutions, since no tensioning of the medium from the front part of the printer is needed to control the cockle effect.
Another advantage of avoiding the use of tensioning devices is that there is no appreciable difference in the drive between the first passes and the rest of the printing operation; on the contrary, when a tensioning device is used the advance of the paper in the first passes can be different from the advance once the paper is engaged by the tensioning device, causing differences in the plot.
In case of vacuum holddown devices, the avoidance of overdrive wheels also simplifies the construction of the vacuum system and minimises its power losses, since there are no mechanical parts of the driving system housed in the vacuum conduits. Therefore, the power consumption of the holddown system is also reduced, and the level of noise caused by the vacuum system is also lower.
In the preferred embodiment of the invention, said first cockle-control means cause at least some of the bubbles to expand downwards into a plurality of front vacuum channels of the supporting platen which extend at least in the output zone.
Bubbles and wrinkles due to cockle grow downwards into the front vacuum channels instead of growing upwards towards the printhead: the risk of ink smearing or paper crash is thus further reduced.
Moreover, since most of the expansion of the medium can be controlled to grow downwards instead of upwards, it is possible to reduce the height of the printheads on the medium (pen-to-paper spacing), thus improving the quality of the plot.
In advantageous embodiments, the distance between centres of adjacent front channels of the platen is between 8 and 20 mm, preferably about 13 mm.
This geometry induces a satisfactory wave form of the deformed paper, with bubble heights than cannot cause ink smearing.
This preferred values of the spacing between channels have been selected on the base of the media generally used in this type of printers, but for other kinds of media the optimum spacing may be different; in general, for thicker or stiffer media the distance between channels should be larger, while for thinner and more flexible media, the distance should be smaller.
In one embodiment, said front vacuum channels formed in said supporting platen extend partly in the print zone and partly in the media output zone; preferably, a first portion of the front vacuum channels extending between the print zone and the first part of the output zone widens progressively in the direction of advance of the medium.
The position and geometry of the front vacuum channels allow the growth of the bubbles inside them taking into account their progressive expansion, and prevent bubbles in the output zone from travelling towards the print zone in an uncontrolled way.
In further embodiments of the apparatus, the front vacuum channels may comprise a second portion which narrows with respect to the first portion, and a third portion which is wider than said second portion.
In order to improve the sealing of the vacuum system, the walls of at least one of the portions of the front vacuum channels may be at least partly sloped.
According to another aspect of the invention, an inkjet printing apparatus provided with a holddown device for a medium lying on a media supporting platen on which a print zone is defined, is characterised in that said holddown device comprises second cockle-control means which control an expansion of the medium in the print zone to be in the form of at least two parallel waves defined by a plurality of bubbles, said waves being alternated such that a downward bubble of one of the waves is adjacent to an upward bubble, or no bubble, of an adjacent wave in the direction of advance of the medium.
This expansion in alternate waves in the print zone compensates the positioning errors of the drops of ink that may occur if the medium expands in the print zone forming a uniform wave in the direction of advance of the medium; thus, defects of vertical banding in the plot are avoided.
Preferably, said second cockle-control means comprise a plurality of rear vacuum channels extending at least in the initial part of the print zone and a plurality of front vacuum channels extending at least in the final part of the print zone, said rear vacuum channels and said front vacuum channels being arranged alternated along a scan direction at right angles to said direction of advance of the medium.
The rear vacuum channels extend the vacuum towards the very first part of the printing zone, and the alternance of rear and front channels cause the medium to be deformed as explained in order to avoid vertical banding.
According to a preferred embodiment, the inkjet printing apparatus of the present invention comprises both first cockle-control means and second cockle-control means as defined above.
This combination allows cockle to be controlled with the advantages of simple construction and low cost mentioned above and at the same time avoiding vertical banding in the plot for the vast majority of print modes and types of media.
The present invention also proposes a method for holding down a medium being printed in an inkjet printing apparatus, comprising the step of controlling the cockle expansion of the medium, at least in a medium output zone arranged downstream of a print zone in the direction of advance of the medium, to be in the form of a wave defined by a plurality of bubbles, said wave being substantially adapted in frequence to a ridged surface of a supporting platen.
Preferably the method further comprises the step of inducing at least some of the bubbles to grow downwards in front vacuum channels of the supporting platen.
In an advantageous embodiment, the method comprises the step of controlling the expansion of the medium in the print zone to be in the form of at least two parallel waves defined by a plurality of bubbles, said waves being alternated such that a downward bubble of one of the waves is adjacent an upward bubble, or no bubble, of an adjacent wave in the direction of advance of the medium.